This is possibly the smallest smoke detector available. The body is based around a sandwich of the sounder and detection cavities with the control PCB and battery taking up the rest of the space. The resin covered area seems to be for protection against humidity. The covered circuitry is the sensor amplifier and the control chip (probably a microcontroller) which may just be standard SMD devices.
I forgot to mention that the test button on the side also acts as a mute button in the event of a false alarm.
Disassembly of these is pretty much a one-way trip due to the gluing of the sounder and detection areas.
It's a good idea to replace smoke detectors every 5 to 10 years depending on how much dust they get exposed to.
It appears the Cavius range also has a heat detector version which uses a thermistor to detect a sudden increase in heat or an upper alarm threshold.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.